Back to EveryPatent.com
| United States Patent |
5,122,546
|
|
Lee
|
June 16, 1992
|
Polyimide foam precursor
Abstract
A slurry is produced by adding to a polyimide precursor comprising
carboxylic and diamine components at least one foam-enhancing polar,
protic additive of the formula ROH, where R is hydrogen, or C.sub.1 to
C.sub.12 linear or branched alkyl or cycloalkyl, affording a slurry. The
slurry is then brought to a temperature in the range of about 40.degree.
C. to about 105.degree. C., producing a homogeneous, transparent solution,
opaque suspension, or melt. When heated to a higher temperature the melt
foams and cures.
| Inventors:
|
Lee; Raymond (Baton Rouge, LA)
|
| Assignee:
|
Ethyl Corporation (Richmond, VA)
|
| Appl. No.:
|
652865 |
| Filed:
|
February 8, 1991 |
| Current U.S. Class: |
521/88; 521/117; 521/183; 521/184; 521/185; 521/189 |
| Intern'l Class: |
C08J 009/14 |
| Field of Search: |
521/183,184,185,189,88,117
|
References Cited
U.S. Patent Documents
| Re30213 | Feb., 1980 | Gagliani et al. | 521/185.
|
| 3506583 | Apr., 1970 | Boram et al. | 252/188.
|
| 3554939 | Jan., 1971 | Lavin et al. | 260/2.
|
| 4153783 | May., 1979 | Gagliani et al. | 528/337.
|
| 4161477 | Jul., 1979 | Long et al. | 260/326.
|
| 4183838 | Jan., 1980 | Gagliani | 260/32.
|
| 4183839 | Jan., 1980 | Gagliani | 260/37.
|
| 4296208 | Oct., 1981 | Gagliani et al. | 521/77.
|
| 4305796 | Feb., 1985 | Gagliani et al. | 204/159.
|
| 4353998 | Oct., 1982 | Gagliani et al. | 523/219.
|
| 4439381 | Mar., 1984 | Gagliani et al. | 264/26.
|
| 4476254 | Oct., 1984 | Long et al. | 521/180.
|
| 4506038 | Mar., 1985 | Gagliani et al. | 521/103.
|
| 4518717 | May., 1985 | Long et al. | 521/109.
|
| 4599365 | Jul., 1986 | Gagliani et al. | 521/189.
|
| 4621015 | Nov., 1986 | Long et al. | 521/183.
|
| 4647597 | Mar., 1987 | Shulman et al. | 521/185.
|
| 4656198 | Apr., 1987 | Shulman et al. | 521/56.
|
| 4855332 | Aug., 1989 | Indyke | 521/184.
|
| 4900761 | Feb., 1990 | Lee et al. | 521/184.
|
| 4913759 | Apr., 1990 | Wright | 156/238.
|
| 4978692 | Dec., 1990 | Ezawa et al. | 521/185.
|
Other References
Final Report NAS 9-14718, "Fire Resistant Resilient Foams", Feb. 1976,
Gagliani.
Final Report NAS 9-15050, "Development of Fire-Resistant, Low Smoke
Generating, Thermally Stable End Items for Aircraft and Spacecraft", Jun.
1977, Gagliani et al.
Final Report NAS -15484, "Development of Fire-Resistant, Low Smoke
Generating Thermally Stable End Items for Commercial Aircraft and
Spacecraft Using a Basic Polyimide Resin" Dec. 15, 1977 to Apr. 15, 1980,
Gagliani, et al.
Final Report NAS-16009, "Formulation and Characterization for Polyimide
Resilient Foams of Various Densities for Aircraft Seating Applications".
|
Primary Examiner: Foelak; Morton
Attorney, Agent or Firm: Hammond; Richard J.
Parent Case Text
This is a continuation-in-part of application Ser. No. 07/575,982, filed
Aug. 30, 1990, which is a continuation-in-part of application Ser. No.
07/466,122, now abandoned filed Jan. 16, 1990.
Claims
I claim:
1. An expandable composition from which polyimide foam can be produced,
said composition comprising a slurry which includes substantially
equimolar amounts of at least one tetracarboxylic component consisting of
at least one lower alkyl di-half-ester of a tetracarboxylic acid of the
formula
##STR9##
wherein A is an organic group and at least one diamine component together
with a volatile polar protic foam-enhancing additive of the formula
ROH
wherein R is selected from hydrogen and C.sub.1 to C.sub.12 linear or
branched alkyl or cycloalkyl which can be unsubstituted or substituted
with halo, aryl, alkoxy and hydroxy, all at a temperature sufficient to
produce a viscous, homogeneous, transparent solution or opaque suspension,
but not cause foaming.
2. The expandable composition of claim 1 in which A is of the formula
##STR10##
wherein X is selected from
##STR11##
3. The expandable composition of claim 2 wherein the carboxylic component
is selected from one or more of the lower alkyl di-half-esters of
benzophenone tetracarboxylic acid; 1,2,3,4-butanetetracarboxylic acid;
oxydiphthalic acid; and diphthalic acid.
4. The expandable composition of claim 1 wherein said diamine component
includes at least one diamine of the formula
H.sub.2 N--R'--NH.sub.2
wherein R' is an organic group.
5. The expandable composition of claim 4 in which R' is selected from
##STR12##
where X is selected from
##STR13##
6. The expandable composition of claim 4 wherein the diamine component is
selected from methylene dianiline; 2,6-dimethylpyridine;
diaminopropylpolydimethylsiloxane;
2,2-bis[4-(4-aminophenoxy)phenyl]-propane; hexamethylenediamine;
1-(2-aminoethyl)piperazine; metaphenylenediamine; polyoxypropylenediamine;
polyoxyethylenediamine; and 2-methylpentamethylenediamine.
7. The expandable composition of claim 1 wherein said polar protic
foam-enhancing additive is selected from water and lower alkyl alcohols.
8. The expandable composition of claim 1 wherein said temperature is within
the range of about 40.degree. C. to about 105.degree. C.
9. The expandable composition of claim 8 wherein said temperature is within
the range of about 40.degree. C. to about 60.degree. C.
10. The expandable composition of claim 9 wherein the viscosity of said
melt is about 5.times.10.sup.6 cps to about 16.times.10.sup.7 cps.
11. The expandable composition of claim 1 wherein the ratio by weight of
the polar protic foam-enhancing additive to the carboxylic and diamine
components combined is between about 1/200 and 1/2.
12. The expandable composition of claim 1 wherein the free volatiles in the
metal comprise about 4 wt. % to about 15 wt. %.
13. The expandable composition of claim 1 wherein the total volatiles
content of the melt is between about 10 wt. % and 40 wt. %.
Description
This invention relates to polyimide foams, especially to the production of
such foams from a precursor melt, as well as the melt itself. Polyimide
foams are useful in fire-resistant thermal insulation and other
applications.
BACKGROUND
Historically, there have been three general ways to produce polyimide
foams. According to the process described in Lavin et al. U.S. Pat. No.
3,554,939, a monomer mixture composed of an ester of benzophenone
tetracarboxylic acid and a polyamine, the mixture having a volatile
content (defined as percent weight loss in 10 minutes at 300.degree. C.)
of at least 9%, is heated to a critical temperature at which foaming
occurs contemporaneously with polymerization of the tetracarboxylic and
polyamine components until the polyimide foam is formed.
In another procedure, described by Gagliani in Final Report NAS 9-14718
entitled "Fire Resistant Resilient Foams" dated February 1976, a mixture
of diamines is added to an alcoholic solution of the half ester of
benzophenone tetracarboxylic acid and reacted at 158.degree.-167.degree.
F. (70.degree.-75.degree. C.) to form a heavy syrup. This syrup is heated
in a circulating air oven at 180.degree. F. (82.2.degree. C.) for about
12-16 hours and the in a vacuum oven at 176.degree.-194.degree. F.
(80.degree.-90.degree. C.) for 60-90 minutes, producing a polyimide
precursor. Thereafter, the polyimide precursor is pulverized into a powder
which is spread over aluminum foil on an aluminum plate and heated at
600.F (315.6.C) in an oven for 30 minutes to produce the foam. In a
similar procedure reported by Gagliani et al. in Final Report NAS 9-15050
entitled "Development of Fire-Resistant, Low Smoke Generating, Thermally
Stable End Items for Aircraft and Spacecraft" dated June 1977, the dried
precursor powder formed in about the same manner was subjected, inter
alia, to multi-stage heating, in which the powder was placed in a pressure
vessel positioned within an oven preheated at 450.degree. F. (232.degree.
C.) and held at this temperature at a reduced pressure (19.9-9.9 inches of
Hg) for I5-30 minutes. The resulting foam was then postcured at
600.degree. F. (315.6.degree. C.) for 15-30 minutes in a circulating air
oven.
The third procedure involves use of microwave radiation for converting the
polyimide precursor into a cellular structure which normally is then
subjected to final curing in a thermal oven. In actual practice the
precursor is used in the form of a powder produced by spray drying an
alcoholic solution of the tetracarboxylic and diamine components. See, for
example, Gagliani et al. U.S. Pat. Nos. 4,296,208; 4,305,796; 4,439,381;
and 4,599,365; Final Report NAS 9-15050 (supra); Final Report NAS 9-15484
entitled "Development of Fire-Resistant, Low Smoke Generating, Thermally
Stable End Items for Commercial Aircraft and Spacecraft Using a Basic
Polyimide Resin" and Final Report NAS 9-16009 entitled "Formulation and
Characterization of Polyimide Resilient Foams of Various Densities for
Aircraft Seating Applications". In U.S. Pat. Nos. 4,305,796 and 4,439,381
it is indicated that the polyimide precursor may vary from a `liquid
resin` to a spreadable, paste-like formulation, depending upon the nature
and quantity of any fillers added to the resin.
Precursors which lead to polyimide foam upon heating have been described in
various terms. U.S. Pat. No. 3,506,583, which issued in 1970, refers to
the precursor as a substantially monomeric "solid solution resinoid." In
U.S. Pat. No. 4,900,761 the mixture of substantially equal molar amounts
of tetracarboxylic component (generally di-half ester) and diamine
component is termed a "paste resin" if the solids content is high and a
"liquid resin" if more of the esterifying alcohol is present. U.S. Pat.
No. 4,153,783 teaches that free volatiles, e.g., alcohol or water, are
detrimental to the foaming process.
The polyimide precursor, regardless of its denomination, has generally been
pulverized or ground into a powder. If sufficient alcohol esterifying
agent is present or can be added, it has been spray-dried, producing a
powder. The powder is then spread as evenly as possible into a mold or
onto a substantially flat, horizontal surface which is then heated to foam
and finally imidize the powder. However, working with the precursor powder
can be hazardous to workmen, because the powder contains amines, many of
which are toxic, and the powder fines are easily airborne. In addition, it
is difficult to spread the powder uniformly into a mold or onto a surface.
It is known that the foaming of polyimides is related to volatile
by-products generated in the amidization and/or imidization reactions.
According to U.S. Pat. No. 4,900,761 small changes in the solids content
of the polyimide precursor lead to large changes in the density of the
resultant foam, but the exemplified changes are random and unpredictable.
Application Ser. No. 07/466,122 discloses a process for making polyimide
foam whereby a polar protic foam-enhancing material is added to a
substantially equimolar mixture of carboxylic and diamine components; the
amount added is between about 1/200 and 1/2 parts by weight of the sum of
the diamine and carboxylic components. The resultant mixture is termed a
slurry. The slurry can be heated, producing a homogeneous "melt," which
can then be foamed. Copending application Ser. No. 07/575,982 discloses
foams in a range of densities which are produced by controlling the amount
of the polar protic foam enhancing-additive which is present in the
slurry. The subject matter of the instant application is related to that
of the these two earlier applications.
SUMMARY OF THE INVENTION
Accordingly, it is an object of this invention to provide a "melt", also
referred to herein as expandable, composition, which can be handled safely
in the workplace, which makes it possible to avoid precursor powder all
together, which can be introduced uniformly and reproducibly into a mold
or other container for foaming, and from which polyimide foam is readily
produced. The melt itself is a unique composition of matter that exists
only within a certain range of temperatures.
According to the present invention, a polar protic foam-enhancing additive
is incorporated into a polyimide precursor composition, which includes
substantially equal molar amounts of carboxylic and diamine components,
producing a slurry. The melt comprises a slurry brought to a temperature
sufficient to produce a viscous, homogeneous, transparent solution or
opaque suspension. The maximum temperature of the melt is that temperature
above which a further increase in temperature leads to foaming in a matter
of minutes. The minimum temperature of the melt is the lowest temperature
at which the melt can, in a matter of minutes, be poured from a container
and spread uniformly on a horizontal surface. In general, this latter
temperature is about 40.degree. C. to about 60.degree. C., in which range
the viscosity of a melt is about 5.times.10.sup.6 cps to about
16.times.10.sup.7 cps.
The polar protic foam-enhancing additive has the formula ROH, where R is
hydrogen, or a C.sub.1 to C.sub.12 linear or branched alkyl or cycloalkyl
radical, which may be unsubstituted or substituted with halo, aryl, alkoxy
and hydroxy. The polar protic foam-enhancing additive need not be miscible
with or act as a solvent for any of the components of the precursor
composition under ambient, i.e. room temperature, conditions. However,
when the additive is heated with the polyimide precursor to a temperature
below that of foam production, the mixture first begins to soften,
dissolve and ultimately forms a homogeneous, transparent solution, opaque
suspension, or "melt." At this stage, the temperature of the melt
ordinarily should not exceed about 105.degree. C. in order to avoid
foaming. The temperature at which the melt exists is in the range of about
40.degree. to about 105.degree. C., most preferably about 40.degree. to
about 60.degree. C.
DETAILED DESCRIPTION
The polar protic foam-enhancing additives ROH include water, i.e., where R
is hydrogen, and also alcohols, as well as polyols, such as glycols, any
of which can be utilized individually or in combination. Thus, when R is
C.sub.1 to C.sub.12 linear or branched alkyl or cycloalkyl, these
additives include one or more of the lower alcohols, such as methanol,
ethanol, or cyclohexanol, as well as those with more complexity, such as
1-octanol, 2-methyldcan-1-ol, 2-ethylhexan-1-ol, cyclopentanol,
cyclohexanol and the like, which can be used in combination. Preferably,
such alkyl groups are C.sub.1 to C.sub.6 linear or branched alkyl, i.e.,
lower alkyl. One requirement of the polar protic foam-enhancing additive
is that it is volatile under the conditions used to produce foam. The term
"volatile" as used herein means the vapor pressure of the additive is high
enough to affect the foam density under the foaming conditions employed.
The amount of the polar protic foam-enhancing additive in a slurry is
equal to the "free volatiles" content of the slurry, measured as described
hereinafter.
In the aforesaid additives, the preferred alkyl R groups may be substituted
with halo, aryl, alkoxy or hydroxy. Illustrative of the halo-substituted
polar protic foam-enhancing additives are 2-chloroethanol,
3-chloropropanol, 4-chlorobutanol and the like. Alkoxy as well as aryl
radicals may also be substituted on the alkyl R groups and include
phenylmethyl (benzyl), 2-phenylethyl (dihydrocinnamyl), methoxymethyl,
ethoxymethyl and the like. Where the substituent on R is a hydroxy group,
the alkyl hydroxides are typically called glycols, and such include
ethylene glycol, propylene glycol, 1,6-hexandiol and the like.
Typically, the polar protic foam-enhancing additive is added to the
polyimide precursor at a ratio between about 200 parts by weight precursor
to 1 part by weight ROH and about 2 parts by weight precursor to 1 part by
weight ROH. Preferably these ratios are from about 100 to 1 to about 5 to
1, most preferably from about 75 to 1 to about 7 to 1. The "melt" referred
to herein comprises a heated slurry of a polyimide precursor, which can
be, but need not be, a particulate, e.g., spray-dried, polyimide precursor
powder, in combination with a polar protic foam-enhancing additive. The
polyimide precursor comprises one or more tetracarboxylic components and
one or more primary, di-, or polyamine components.
Preferably the tetracarboxylic components are esters, predominantly
diesters (half esters) of aromatic tetracarboxylic acids and lower
alkanols such as methanol, ethanol, isopropanol, propanol, the butanols,
the pentanols, the hexanols, and the like. The primary polyamine
components employed are preferably aromatic diamines, heterocyclic
diamines, or combinations thereof, optionally with a minor proportion of
one or more aliphatic diamines.
The organic tetracarboxylic acids or derivatives thereof are preferably
based on tetracarboxylic acids having the general formula:
##STR1##
wherein A is a tetravalent organic group. Although A can be an aliphatic
group, such as the following butyl group, it is
##STR2##
preferred that the organic group A be an aromatic group having a structure
such as the following, it being understood the structures shown are
illustrative, but not limiting:
##STR3##
wherein X is one or more the following:
##STR4##
although other aromatic groups are suitable. The tetracarboxylic acid
derivatives which may be employed include anhydrides, acid halides,
esters, and the like. Of these, esters are preferred and are most
generally used for foam production.
Preferred among the tetracarboxylic acid esters are the alkyl esters of
3,3',4,4'-benzophenone tetracarboxylic acid, most preferably the lower
(C.sub.1 to C.sub.6 linear or branched chain) alkyl, e.g., methyl or
ethyl, diesters thereof. Mixtures of two or more aromatic esters, most
preferably predominating in diesters, may be employed, if desired.
It is also possible, in accordance with this invention, that the
tetracarboxylic component employed in the manufacture of the polyimide
foams be a caprolactam derivative as disclosed in U.S. Pat. Nos.
4,161,477, 4,183,838 and 4,183,839, the contents of which are incorporated
herein by reference.
The tetracarboxylic component may also be an N-substituted imido acid ester
of the tetracarboxylic acid as disclosed in U.S. Pat. Nos. 4,647,597 and
4,656,198, the contents of which are incorporated herein by reference.
The organic diamine component of the polyimide precursor may be represented
by the formula:
H.sub.2 N13 R'--NH.sub.2
wherein R' is an aromatic group containing about 5 to 16 carbon atoms and
which may contain at least one hetero atom in the aromatic ring, the
hetero atom being nitrogen, oxygen or sulfur. Also included are aromatic
groups such as the following, it being understood the structures shown are
illustrative, but not limiting:
##STR5##
where X is selected from
##STR6##
Representatives of such diamines include: 2,6-diaminopyridine
3,5-diaminopyridine;
3,3'-diaminodiphenylsulfone;
4,4'-diaminodiphenylsulfone;
4,4'-diaminodiphenylsulfide;
3,3'-diaminodiphenylether;
4,4'-diaminodiphenylether;
metha-phenylenediamine;
para-phenylenediamine;
4,4'-methylene dianiline;
2,6-diamino toluene;
2,4-diamino toluene;
2,2-bis[4-(4-aminophenoxy)phenyl]propane
4,4'-[1,4-phenylene(1-methylethylidene)]bis(benzenamine)
and the like.
It is also possible and sometimes desirable in the preparation of the
polyimide precursors, to include in the reaction mixture one or more
aliphatic diamines. Such aliphatic diamines are preferably alpha-omega
diaminoalkanes having the formula:
H.sub.2 N--(CH.sub.2).sub.n --NH.sub.2
wherein n is an integer from 2 to 16. Representatives of such diamines
include 1,3-diaminopropane, 1,4-diaminobutane, 1,5-diaminopentane,
1,6-diaminohexane, etc. Alternatively, heteroaliphatic diamines, such as
1-(2-aminoethyl)piperizine, can be employed.
In place of or in addition to the foregoing aliphatic amines, use can be
made of aliphatic etherified polyamines such as polyoxypropylene diamines
having the formula:
H.sub.2 N--CH(CH.sub.3)CH.sub.2 --[OCH.sub.2 CH(CH.sub.3)].sub.x --NH.sub.2
wherein x varies from about 1 to about 5. Polyoxyethylenediamines are also
useful. A number of aliphatic etherified polyamines, e.g., Jeffamine
EDR-148, T-403, D-2000, and T-3000, are available from Jefferson Chemical
Co.
Other useful primary diamines which may be included in the polyimide
precursor include amino-terminated butadiene-nitrile copolymers having the
general formula:
##STR7##
wherein R is either a phenylene group or an alkylene group, R.sub.1 is
hydrogen or methyl, and x and y are each independently integers ranging
from 1 to 25 and n is an integer, preferably below 20. In these copolymers
it is preferred that butadiene constitute at least 50% by weight of the
butadiene and nitrile monomers. The nitrile monomer copolymerized with the
butadiene can either be acrylonitrile or methacrylonitrile. Such
copolymers generally have low molecular weights, preferably less than
3,000, to insure that they are sufficiently fluid to react in the
formation of the polyimide as well as sufficiently fluid so as to be
capable of foaming.
Still another type of primary diamine component which may be included in
the polyimide precursor are aromatic amino-terminated silicones, such as
those having the general formula:
##STR8##
wherein R is a C.sub.2 to C.sub.6 alkylene group, R.sub.1 and R.sub.2 are
each independently lower alkyl containing 1 to 3 carbon atoms and n is an
integer from 1 to 4. Another useful siloxane is diaminopropylpolydimethyl
siloxane, which is available from General Electric Co. as "G-10."
Another preferred category of diamines which may be utilized in the
polyimide precursor are the diesters produced from amino-substituted
aromatic carboxylic acids and polymethylene glycols. Such diesters may be
represented by the general formula:
H.sub.2 N--ArCOO--R--OOCAr--NH.sub.2
wherein R is an alkylene group (which may be branched or straight chain)
and which preferably contains from 3 to 8 carbon atoms, most preferably
trimethylene; and Ar is an aromatic group which may be composed of one or
more fused or non-fused benzene rings which in turn may carry suitable
substituents (e.g., nitro, alkoxy, etc.) in addition to the primary amino
groups. A few exemplary diesters of this type include:
ethylene glycol-4-aminobenzoic acid diester;
ethylene glycol-3-aminobenzoic acid diester;
ethylene glycol-2-aminobenzoic acid diester;
trimethylene glycol-3-aminobenzoic acid diester;
trimethylene glycol-2-aminobenzoic acid diester;
trimethylene glycol-3-amino-2-nitrobenzoic acid diester;
tetramethylene glycol-3-amino-4-nitrobenzoic acid diester;
tetramethylene glycol-3-amino-5-nitrobenzoic acid diester
tetramethylene glycol-4-amino-2-nitrobenzoic acid diester;
1,5-pentanediol-4-amino-3-nitrobenzoic acid diester;
1,6-hexanediol-5-amino-2-nitrobenzoic acid diester;
neopentyl glycol-4-amino-2-methylbenzoic acid diester;
1,8-octanediol-4-amino-2-propylbenzoic acid diester;
1,9-nonanediol-3-amino-4-methylbenzoic acid diester;
1,10-decanediol-4-(4-aminophenyl)benzoic acid diester;
and the like. Mixtures of such diesters may be employed.
A particularly preferred diester of this type is the diester of
trimethylene glycol (1,3-propanediol) and 4-aminobenzoio acid.
The relative proportions of the reactants used in the preparation of the
polyimide precursor can be varied. In general, it is preferred to employ
essentially stoichiometric proportions of the tetracarboxylic component
and the primary diamine component.
When using a lower alkyl ester of the tetracarboxylic acid, either or both
the alcohol produced and the water released during the
amidization/imidization reactions, can, in addition to the polar protic
foam-enhancing additive, function as the blowing agent during
polymerization to form the desired polyimide foams. In addition, use can
be made of any of a variety of well-known organic or inorganic blowing
agents which can be introduced separately. Variations in the concentration
of the blowing agent can be used to achieve specific densities and ILD
(indention load deflection) values. However, the use of ROH, as defined
above, is preferred as the sole blowing agent, since the foams produced
thereby are superior.
The polyimide precursor may also contain various filler and/or reinforcing
materials. For example, graphite, glass and other synthetic fibers can be
added to the precursor composition to produce a fiber-reinforced product.
Glass or phenolic micro balloons may be added for density adjustment, if
desired, but the microballoons increase density at the expense of
flexibility.
It is frequently desirable to also add one or more surfactants to the
polyimide precursor, thereby increasing cell stability and uniformity,
increasing fatigue resistance, and also making the foam more flexible and
resilient. The nature of such surfactants for this use is well known and
reported in the patent literature.
When producing foams from tetracarboxylic components other than lower alkyl
esters, a blowing agent and/or microballoons generally have been employed
in order to achieve a suitable cellular structure.
Although not necessary, for some applications it is desirable that the
polyimide precursor contain a suitable quantity of a flame retardant
material in order to still further increase the flame resistance of the
foam.
Polyimide foams of good quality are produced from the melt obtained by
combining a polyimide precursor with the foam-enhancing additive,
producing a slurry, and then heating the slurry at not more than about
105.degree. C. Thereafter, the melt is heated at one or more temperatures
sufficient to obtain a consolidated but friable cellular foam structure,
and in at least one other stage this cellular foam structure is heated at
one or more higher temperatures sufficient to cure the cellular material
into a resilient polyimide foam. Preferably, these stages are conducted in
a continuous manner as by supporting the material being foamed on a moving
belt or rotating platform associated with appropriate heating apparatus
maintained at suitable temperatures (e.g., one or more tunnel heaters,
etc., with appropriate temperature zones along the path of travel).
Alternatively, natively, a single furnace, oven or other thermal apparatus
is employed whereby the material being foamed is kept more or less in one
place, and the heat applied thereto is suitably increased during the
period of time the melt, developing cellular structure and developed
cellular structure are maintained in the apparatus. Any type of apparatus
may be used for applying the thermal energy to the melt, to the cellular
structure as it is developing therefrom, and to the resultant developed
cellular structure. Such equipment includes radiant heaters; furnaces
operated on natural gas, LPG, fuel oil, etc.; dielectric heaters;
microwave cavities; and the like. However, use of microwave or thermal
ovens (e.g., a single circulating air electric oven operated such that its
temperature is appropriately increased during the residency of the
material being processed therein, or two or more electric resistance
heaters positioned in and along the length of a tunnel or cavity with
their temperatures suitably set or regulated such that the material being
conveyed therethrough encounters increased temperatures during its travel)
are preferred because of the lower capital and operating costs involved
when using such apparatus. The cured polyimide foam may of course be
subjected to a final postcuring at still higher temperatures if desired.
In most cases the foam structure is produced from the melt at temperatures
within the range of about 120.degree. to about 180.degree. C., and
preferably in the range of about 135.degree. to about 170.degree. C.
(about 275.degree. to about 325.degree. F.), and curing is effected at
temperatures of at least about 220.degree. C. (preferably at least about
230.degree. C.). However, departures from these ranges are permissible
where the circumstances warrant or justify such departures. Usually
temperatures above about 425.degree. C. are not used, as thermal
degradation of the foam may be encountered, depending of course on the
composition of the foam being processed, some foams having greater thermal
stability than others.
Any of a number of procedures may be used for forming the melt composition.
For example, the tetracarboxylic and diamine components may be mixed with
a suitable solvent or liquid diluent and the polar protic foam-enhancing
agent in appropriate proportions to form the slurry directly. Similarly, a
more dilute solution of the components may be concentrated to the desired
solids content as by use of vacuum distillation at a suitably low
temperature such that excessive reaction between the tetracarboxylic and
diamine components does not occur. Generally speaking, melt compositions
with a solids content in the range of about 60 to about 90 weight percent,
i.e., a total volatiles content of about 40 to about 10 wt. %, are
preferred, and those with a solids content in the range of about 65 to
about 85 weight percent are particularly preferred.
Having described the basic concepts of this invention, reference is now
made to the following examples which serve to still further illustrate the
practice and advantages of this invention. Melt viscosities ("vis" below)
which appear in this application are determined with a Brookfield
viscometer, Series RVT using a T-bar spindle. "Total volatiles", wherever
used in this application, is the percent weight lost from a sample heated
for 30 minutes at a temperature of 260.degree. C. It is believed to
include both the free volatiles, such as solvent, as well as the bound
volatiles which arise from amidization and imidization reactions. "Free
volatiles", wherever used in this application, is the percent weight lost
from a sample heated at 50.degree. C. under vacuum (30 in. Hg) for 3 hr.
Free volatiles is equivalent to the wt. % of the polar protic
foam-enhancing additive in the sample.
EXAMPLE 1
To a 1.0 liter three-neck, round-bottom glass flask, equipped with a
mechanical stirrer, thermometer, heating jacket, and condenser, is added
320 mL (8.0 moles) methyl alcohol, and 24 mL (1.33 moles) distilled water.
The solution is thoroughly mixed and 322.23 g (1.0 mole)
benzophenonetetracarboxylic acid dianhydride (BTDA) are added with
stirring. This mixture is then heated to reflux to form the methyl ester
of the BTDA. Once the esterification reaction is complete, the clear
solution is cooled to below 40.degree. C. and 158.6 g (0.8 mole) of
methylene dianaline (MDA) are added. After the MDA has completely
dissolved, 21.8 g (0.2 mole) of diaminopyridine (DAP) are added and mixed
until dissolved. This is followed by the addition of 7.0 grams (1.5
percent by weight of the polyimide solids) of a silicone glycol
surfactant, Dow Corning 193. The solution is thoroughly mixed to yield a
liquid polyimide precursor.
The liquid polyimide precursor is processed into powder, i.e., a
particulate material, using a spray dryer or a vacuum dryer.
EXAMPLE 2
About 300 g of polyimide precursor (prepared as described in Example 1) was
placed in a "sigma-blade" mixer, along with an appropriate amount of water
and heated to about 55.degree. C. The mixer was then turned on and the
materials mixed (about 1 hour) until a homogeneous paste, resembling
taffy, was obtained. The melt was then poured out onto a heat resistant,
microwave compatible (e.g. teflon coated glass cloth) substrate. The melt
was then placed in a Gerling-Moore laboratory microwave oven operating at
a frequency of 2450 mHz for 20 minutes at 200 watts for preheating. Once
the melt was thoroughly preheated the melt was foamed for 20 minutes at
220 watts followed by curing for 20 minutes at 2200 watts followed by 60
minute thermal post-cure at 500.degree. F. A low density, resilient
polyimide foam is obtained.
The effect of varying the amount of water added is described in Table 1.
TABLE 1
______________________________________
Conc Water % Water by Karl
Den- Tensile.sup.b
Thermal.sup.c
Added (%) Fisher Analysis
sity.sup.a
Strength
K.
______________________________________
0 (Powder only)
0.8 0.68 12.3 0.28
3 3.1 0.51 11.1 0.30
5 4.1 0.46 6.5 0.34
______________________________________
.sup.a lb/ft.sup.3
.sup.b psi
.sup.c (BTU .multidot. in)/hr .multidot. ft.sup.2 .multidot. .degree.F.
EXAMPLE 3
A melt composition, having a solids content of 65-70 percent and a
surfactant concentration of 4.0 percent, was prepared using the method
described in Example 2 and was mixed with varying amounts of carbon
fibers, ranging in concentration from 0.5-50 percent. These filled resins
were then foamed and cured as described in Example 2 to obtain a series of
reinforced foams. It was found that as the concentration of fiber
increased the density of the foam increased, and the foam became more
rigid.
EXAMPLE 4
In a 12 1 three neck flask, equipped with a stirrer, 4.189 kg of
3,3',4,4'-benzophenone tetracarboxylic acid (BTDA), 1.440 kg of CH.sub.3
OH and 90 g of water were heated to reflux. Once the solution became clear
heating was continued for an additional hour to insure complete conversion
to the diester derivative. The diester solution was transferred to a 1
gallon capacity "sigma-blade" mixer, and 1.838 kg of 4,4'-methylene
dianiline (MDA) was added and mixed for 15 minutes. Then 434 g of
2,6-diaminopyridine (DAP) was added and the heating to jacket set at
60.degree. C. Heating and mixing continued for 26 minutes, temperature at
60.degree. C., and 90.1 g of surfactant was added along with 296 g H.sub.2
O. The resultant melt was mixed for an additional hour.
The melt was then poured out, and four buns (about 1500 g each) foamed in a
Gerling-Moore laboratory microwave oven. The melt was preheated 3 minutes
at 1400 watts, followed by foaming for 30 minutes at 2200 watts, followed
by curing for 15 minutes at 4000 watts and a thermal oven post-cure of 60
minutes at 500.degree. F.
The articles were low density, flexible, resilient polyimide foams.
EXAMPLE 5
An analysis of the powder particulate polyimide precursor prepared in an
identical manner as that of Example 1 revealed that it had a total
volatiles content of 20.6%, with 0.58% of such volatiles being water. The
powder was exposed to the air (ambient temperature and pressure) for 30
minutes. Analysis of the resulting powder gave total volatiles of 20.9%,
with 2.74% being water. A total water gain of 2.16 wt. % therefore
occurred, with total volatiles slightly elevated i.e., a loss in the
volatile components of 1.86 wt. % occurred.
The resulting polyimide precursor powder isolated from the above procedure
was foamed by the same process as shown in Example 4. The polyimide foam
bun produced had a tensile strength of 10.33 psi (average of two); a
density of 0.589 lbs/ft.sup.3 (average of two); and a thermal K of 0.305
(BTU.multidot.in)/(hr.multidot.ft.sup.2 .multidot..degree.F.)
EXAMPLE 6
To 35 lb. quantities of polyimide preoursor powder prepared as described in
Example 1 was added 7to ml of a mixture of various ratios of water and
methanol in a mixer as described in Example 2. The resultant melts were
then foamed as set forth in the latter Example. The results obtained are
set forth in Table 2.
TABLE 2
______________________________________
MEOH H.sub.2 O
Foam density
(ml) (ml) (lb/ft.sup.3)
______________________________________
0 700 0.38
100 600 0.43
200 500 0.44
______________________________________
EXAMPLE 7
A polyimide precursor is prepared in the manner of Example 1, except that
the molar ratio (MDA)/(DAP) is 0.715/0.305 rather than 0.8/0.2, and the
free volatiles content of the powder particulate precursor is less than
about 5 weight percent. Varying amounts of water are added to 500 g
portions of the resultant powder, producing a series of slurries. The
slurries are mixed and heated at 50.degree. C. to 62.degree. C. for 20
minutes, producing a series of melts. The melts are then foamed and cured
by heating the melts first for 30 mins. in a circulating air, convection
oven at 68.degree. C., followed by heating in the Gerling-Moore laboratory
microwave oven of Example 2 for 20 min. at 1400 watts, followed by 40 min.
at 2800 watts. Lastly, the resultant foams are heated in the convection
oven for 30 min. at 260.degree. C. The result is a series of foams with a
range of densities, as shown in Table 3.
TABLE 3
______________________________________
Slurry Cured Foam
wt % % total density tensile
Added H.sub.2 O
volatiles.sup.a
lb/ft.sup.3
psi % LOI.sup.b
thermal K..sup.c
______________________________________
0 19.97 0.54 7.1 37 0.312
3 20.68 0.48 9.9 34 0.290
5 21.23 0.38 6.6 36 0.322
7 21.93 0.28 6.4 36 0.363
9 22.13 0.25 2.1 32 0.373
11 23.60 0.25 2.4 33 0.400
11 25.46 0.21 2.6 32 0.383
13 26.92 0.21 2.0 32 0.408
15 28.09 0.17 30 0.457
17 30.54 0.15 2.0 31 0.424
______________________________________
.sup.a Percent weight lost from sample heated for 30 min. at 260.degree.
C.
.sup.b Limiting Oxygen Index
.sup.c (BTU .multidot. in)/(hr .multidot. ft.sup.2 .multidot. .degree.F.)
EXAMPLE 8
Using the methods and polyimide precursor of Example 7, except that varying
amounts of several alcohols, equimolar in amount to 3.0 wt. % water, are
added to the powder, producing a series of slurries. The slurries are
mixed and heated at 60.degree. C. to 63.degree. C. for 20 minutes,
producing a series of melts which are foamed and cured in the manner of
Example 7. The results are shown in Table 4.
TABLE 4
__________________________________________________________________________
Slurry Cured Foam
wt % % total
density
tensile
ROH Added ROH
volatiles.sup.a
lb/ft.sup.3
psi % LOI.sup.b
thermal K..sup.c
__________________________________________________________________________
HOH 3.0 20.68
0.48
9.9 34 0.290
MeOH 5.33 27.20
0.38
5.3 33 0.342
EtOH 7.67 24.62
0.36
5.0 34 0.339
i-PrOH
9.99 25.54
0.36
7.7 34 0.323
i-BuOH
12.38 26.62
0.32
5.6 33.5
0.372
__________________________________________________________________________
.sup.a Percent weight lost from sample heated for 30 min. at 260.degree.
C.
.sup.b Limiting Oxygen Index
.sup.c (BTU .multidot. in)/(hr .multidot. ft.sup.2 .multidot. .degree.F.)
EXAMPLE 9
1,2,3,4-Butanetetracarboxylic acid dianhydride is reacted with an
eight-fold molar excess of methanol at a temperature of 75.degree. C. for
11 hr, yielding the corresponding dimethyl half-ester (BTCDE). The
dimethyl half-ester (BTDE) of benzophenonetetracarboxylic acid dianhydride
is similarly prepared in excess methanol. Appropriate quantities of the
half esters in methanol are mixed to achieve the desired mole ratios, and
30 g or 300 g thereof is transferred to a 100 ml or 1 L beaker,
respectively. The beaker and its contents are stirred and heated to
75.degree. C. on a hot plate, and methylene dianiline (MDA) is added to
the beaker in an amount which is substantially equimolar to the
dianhydride mixture. While at 75.degree. C., water (5% by weight of the
combined diamine and diester) is added to the beaker, along with a
silicone glycol surfactant (Dow Corning 193) in an amount equal to 2% by
weight of the combined diamine and diester. After stirring, the viscous,
homogeneous melt is removed from the hotplate, placed on a Teflon-coated
glass cloth, smoothed and allowed to cool to room temperature and harden.
The solidified melt is foamed in a 5 kW Gerling-Moore laboratory microwave
oven at a power level of 2.8 kW for 30 min. The foamed specimen is then
placed in a Blue M thermal oven for 30 min. at 260.degree. C. to cure it.
The properties of the melts and foams obtained from them at various
BTCDE/BTDE ratios appear in Table 5.
TABLE 5
______________________________________
BTCDE/BTDE PPFEA.sup.a
Foam D
molar wt % lb/ft.sup.3
Foam Character
______________________________________
0/100 7.82 0.46 flexible, homog.
8/92 5.21 0.40 "
15/85 6.32 0.73 thin cell walls
22/78 4.24 0.96 homog. struct..sup.b
26/74 12.11 "
29/71 7.51 0.97 "
35/65 5.33 filmy cells
62/38 5.70 "
100/0 12.11 "
______________________________________
.sup.a Polar protic foamenhancing additive
.sup.b Used 0.5 wt % surfactant
EXAMPLE 10
Using a procedure like that of Example 9, except that the half-ester (ODPE)
of oxydiphthalic anhydride in methanol is combined with the half-ester of
BTDE, the melts and foams described in Table 6 are obtained.
TABLE 6
______________________________________
ODPE/BTDE PPFEA.sup.a
Foam D
molar wt % lb/ft.sup.3
Foam Character
______________________________________
0/100 8.64 0.75 flexible, resil.
10/90 13.3 0.61 "
21/79 8.63 0.55 "
81/19 7.75 0.44 "
100/0 6.94 0.58 "
______________________________________
.sup.a Polar protic foamenhancing additive
EXAMPLE 11
Using a procedure like that of Example 9, except that the half-ester (DPE)
of diphthalic anhydride in methanol is combined with the half-ester of
BTDE, the melts and foams described in Table 7 are obtained.
TABLE 7
______________________________________
DPE/BTDE PPFEA.sup.a
Foam D
molar wt % lb/ft.sup.3
Foam Character
______________________________________
0/100 6.95 0.75 flexible, stri.
5/95 10.14 0.60 flexible, homog.
11/89 12.33 0.50 flexible, stri.
22/78 10.89 0.63 "
42/58 4.55 brown, brittle
81/19 4.61 collapsed, char
100/0 5.53 "
______________________________________
.sup.a Polar protic foamenhancing additive
EXAMPLE 12
By the method of Example 9, except that BTDE is the sole half-ester
employed, and varying amounts of the MDA are replaced with 10-100 mole %
2,6-dimethylpyridine (DAP), the melts yielded flexible, resilient foam in
all cases.
EXAMPLE 13
By the method of Example 9, except that BTDE is the sole half-ester
employed, and varying amounts of the MDA are replaced with
diaminopropylpolydimenthylsiloxane ("G-10" from General Electric Co.,),
the melts and resultant foams described in Table 8 are obtained.
TABLE 8
______________________________________
G-10/MDA PPFEA.sup.a
Foam D
molar wt % lb/ft.sup.3
Foam Character
______________________________________
0/100 7.52 0.37 flexible, resil.
0.5/99.5 10.51 0.52 "
1.2/98.8 9.25 0.37 "
2.4/97.6 7.14 foam collapsed
5.0/95 12.44 "
10/90 6.48 "
25/75 9.43 "
______________________________________
.sup.a Polar protic foamenhancing additive
EXAMPLE 14
By the method of Example 9, except that BTDE is the sole half-ester
employed, and varying amounts of the MDA are replaced with
2,2-bis[4-(4-aminophenoxy)phenyl]propane (BAPP), the melts and resultant
foams described in Table 10 are obtained.
TABLE 10
______________________________________
BAPP/MDA PPFEA.sup.a
Melt T/Vis Foam D Foam
molar wt % .degree.C./cps
lb/ft.sup.3
Character
______________________________________
100/0 0.57 flexible, resil.
50/50 12.5 50/18 .times. 10.sup.6
0.67 "
40/60 9.9 " 0.23 "
30/70 0.23 "
20/80 7.4 " 0.23 "
______________________________________
.sup.a Polar protic foamenhancing additive
EXAMPLE 15
By the method of Example 9, except that BTDE is the sole half-ester
employed, and varying amounts of the MDA are replaced with
hexamethylenediamine (HMDA) and/or 2,6-diaminopyridine (DAP), the melts
and resultant foams described in Table 11 and 12 are obtained.
TABLE 11
______________________________________
DAP/HMDA/MDA PPFEA.sup.a
Foam D
wt % wt % lb/ft.sup.3
Foam Character
______________________________________
0/13/87 10.3 0.41 flexible, resil.
0/8/92 8.8 0.31 "
6/11/83 12.3 0.37 "
9/8/83 10.0 0.35 "
11/8/81 10.4 "
______________________________________
.sup.a Polar protic foamenhancing additive
TABLE 12
______________________________________
DAP/HMDA/ Foam
MDA PPFEA.sup.a
Melt T/Vis D Foam
molar wt % .degree.C./cps
lb/ft.sup.3
Character
______________________________________
74/17/15 12.6 50/18 .times. 10.sup.6
0.41 flexible, resil.
78/16/18 12.6 " 0.29 "
74/17/15 7.8 " 0.48 "
______________________________________
.sup.a Polar protic foamenhancing additive
EXAMPLE 16
By the method of Example 9, except that BTDE is the sole half-ester
employed, and the diamine component includes 13 wt. % 2,6-diaminopyridine
(DAP), 7 wt. % 1-(2-aminoethyl)piperazine (AEP) and 80 wt. %
methylenedianiline, a melt containing 9.8 wt. % free volatiles (methanol
and water) results, from which a flexible, resilient foam weighting 0.36
lb/ft.sup.3 prepared.
EXAMPLE 17
By the method of example 9, except that BTDE is the sole half-ester
employed, and varying amounts of the MDA are replaced with
metaphenylenediamine (MPA) and/or a polyoxypropylene diamine (T403)
commercially available from Jefferson Chemical Co. as Jeffamine T-403, the
melts and resultant foams described in Table 13 are obtained.
TABLE 13
______________________________________
MDA/MPA/ PPFEA.sup.a
Melt T/Vis Foam D Foam
T403 molar
wt % .degree.C./cps
lb/ft.sup.3
Character
______________________________________
76/15/9 10.5 50/18 .times. 10.sup.6
0.29 flexible, resil.
87/10/3 0.37 "
90/10/0 0.29 "
80/20/0 10.1 " 0.28 "
70/30/0 10.7 " 0.27 "
60/40/0 10.3 " 0.24 "
56/44/0 10.9 " 0.27 "
______________________________________
.sup.a Polar protic foamenhancing additive
EXAMPLE 18
Melts are produced by the method of Example 9, except that BTDE is the sole
half-ester employed, and varying amounts of the following diamines are
reacted: Jeffamine EDR 148 (EDR), Jeffamine T-403 (T403), methylene
dianiline (MDA), metaphenylenediamine(MPA), 1-(2-aminoethyl)piperizine
(AEP), Jeffamine T-3000 (T3K), 2,6-diaminopyridine (DAP),
2-methylpentmethylenediamine (D0Y), and Jeffamine D-2000 (D2K). The melt
compositions and properties appear in Table 14. In each case usable foams
are produced by employing the foaming and curing procedures set forth in
Example 9.
TABLE 14
__________________________________________________________________________
Molar Composition Foam D
BTDE
EDR T403
MDA MPA AEP
T3K
DAP
DOY D2K
lb/ft.sup.3
__________________________________________________________________________
.55 .02 .51 .06 0.51.sup.e
.55 .50 .07 .06
.sup. tr.sup.a
0.36
.55 .08
.47 tr 0.52.sup.f
.55 .03
.47 .07 0.48
.56 .60 0.25
.56 .34 .27 .10 0.33
.72 .61 .01 0.34
.55 .50 .12 0.24
.55 .58 .09 0.27.sup.b
.55 .03
.49 .09 0.28
.89 .05
.81 .11 0.36
.55 .61 0.35
.55 .06
.54 0.39
.55 .61 .01
0.36.sup.c
.55 .61 .01 0.33
.55 .61 .03 0.39
.55 .61 0.30
.55 .59 .01 tr 0.32
.55 .59 .07 tr 0.28
.55 .05 .56 .07 0.32
.55 .16 .48 tr 0.30
.55 .64 tr 0.25.sup.d
.55 .05 .52 .07 tr 0.39
__________________________________________________________________________
.sup.a Trace, i.e., less than 0.01 mole
.sup.b Viscosity = 5.96 .times. 10.sup.6 cps at 41.degree. C.
.sup.c Viscosity = 16 .times. 10.sup.6 cps at 60.degree. C.
.sup.d Viscosity = 6.6 .times. 10.sup.6 cps at 53.degree. C.
.sup.e Viscosity = 15.8 .times. 10.sup.7 cps at 52.degree. C.
.sup.f Viscosity = 5.7 .times. 10.sup.6 cps at 53.degree. C.
EXAMPLE 19
Melts are prepared by the method of Example 9, except that BTDE is the sole
half-ester employed, and varying amounts of MDA and Epon 1061M, which is
4,4'[1,4-phenylene(1-methylethylidene)]bis-(benzeneamine), are employed.
The melts and resultant foams are listed in Table 15.
TABLE 15
______________________________________
MDA/Epon PPFEA.sup.a
Melt T/Vis Foam D Foam
1061M molar
wt % .degree.C./cps
lb/ft.sup.3
Character
______________________________________
92/10 9.1 50/18 .times. 10.sup.6
0.25 flexible, resil.
82/20 0.27 "
71/31 11.1 " 0.30 "
.sup. 61/31.sup.b 0.34 "
51/51 8.8 " 0.36 "
0/102 0.40 "
______________________________________
.sup.a Polar protic foamenhancing additive
.sup.b Also includes 10.2 mole percent BAPP
As can be seen from the foregoing, the melt compositions used in the
practice of this invention comprise a slurry of at least (i) one or more
organic tetracarboxylic compounds, (ii) one or more organic diamine
compounds co-reactive therewith, and (iii) a polar, protic foam-enhancing
additive, all at a temperature lower than the temperature which causes
foaming, but high enough to produce a viscous, homogeneous, transparent
solution or opaque suspension which can be poured from a container or
spread uniformly on a horizontal surface. Preferably the diamines include
at least one aromatic and/or aromatic heterocyclic primary diamine.
Components (i) and (ii) are usually present in the mixture in essentially
stoichiometric (substantially equal molar) quantities. Most preferably,
such melt compositions further include a suitable quantity of a
surfactant, most preferably a silicone glycol surfactant.
It will be apparent that this invention is susceptible to considerable
variation in its practice without departing from the spirit and scope of
the appended claims, the embodiments described hereinabove being merely
exemplary of its practice.
Top